Commits on Oct 31, 2023

1. Add support for pre-unix-epoch file dates on Apple platforms (rust-la… …

   …ng#108277)
   
   Time in UNIX system calls counts from the epoch, 1970-01-01. The timespec
   struct used in various system calls represents this as a number of seconds and
   a number of nanoseconds. Nanoseconds are required to be between 0 and
   999_999_999, because the portion outside that range should be represented in
   the seconds field; if nanoseconds were larger than 999_999_999, the seconds
   field should go up instead.
   
   Suppose you ask for the time 1969-12-31, what time is that? On UNIX systems
   that support times before the epoch, that's seconds=-86400, one day before the
   epoch. But now, suppose you ask for the time 1969-12-31 23:59:00.1. In other
   words, a tenth of a second after one minute before the epoch.  On most UNIX
   systems, that's represented as seconds=-60, nanoseconds=100_000_000. The macOS
   bug is that it returns seconds=-59, nanoseconds=-900_000_000.
   
   While that's in some sense an accurate description of the time (59.9 seconds
   before the epoch), that violates the invariant of the timespec data structure:
   nanoseconds must be between 0 and 999999999. This causes this assertion in the
   Rust standard library.
   
   So, on macOS, if we get a Timespec value with seconds less than or equal to
   zero, and nanoseconds between -999_999_999 and -1 (inclusive), we can add
   1_000_000_000 to the nanoseconds and subtract 1 from the seconds, and then
   convert.  The resulting timespec value is still accepted by macOS, and when fed
   back into the OS, produces the same results. (If you set a file's mtime with
   that timestamp, then read it back, you get back the one with negative
   nanoseconds again.)
   
   Co-authored-by: Josh Triplett <josh@joshtriplett.org>

   

   Byron and joshtriplett committed Oct 31, 2023
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